(1) Field of the Invention
The present invention relates to a sintered alumina article with a high flexural strength and a process for producing same. More particularly, the present invention relates to a sintered alumina article which has an excellent flexural strength and is useful as a highly reliable electronic construction material and medical material, and a process for producing the same under an ambient atmospheric sintering pressure.
(2) Description of the Related Art
Conventional sintered alumina articles which have been prepared by forming corundum (Al.sub.2 O.sub.3) particles into a predetermined shape and then sintering the shaped precursory article under ambient atmospheric pressure exhibits a poor flexural strength of 2000 to 4000 kg/cm.sup.2.
Another conventional sintered alumina article is produced by sintering the shaped precursory article under a high pressure and exhibits an enhanced flexural strength of 6000 to 8000 kg/cm.sup.2, but this high pressure sintering procedure is costly, and thus makes the resultant sintered article expensive. Also, the high pressure sintering procedure restricts the shaping of the sintered article.
A sintered ferrite article, in which sintered ferrite particles are oriented, is now in practical use. The orientation of the ferrite particles provides an anisotropic magnet but not a high strength article. This type of sintered ferrite article is produced by shaping ferrite plate crystalline particles into tapes by a doctor blade method or a calender method, superimposing the tapes on each other, and sintering the superimposed tapes. The plate crystalline ferrite particles can be easily produced by grinding ferrite lumps, and thus the oriented, sintered ferrite article can be easily produced.
Nevertheless, it is very difficult to produce corundum plate crystalline particles by grinding, and thus an oriented, sintered corundum article cannot be easily obtained.
The plate crystalline corundum particles can be produced in accordance with the method disclosed in Japanese Examined Patent Publication No. 37-7750 by a hydrothermal treatment of aluminum oxide particles or aluminum hydroxide particles at a temperature of 100.degree. C. to 300.degree. C. under a pressure of 20 to 100 atmospheres to produce boehmite (AlOOH) particles, and by a further hydrothermal treatment of the boehmite particles with water or an aqueous alkali solution at a temperature of 350.degree. C. or more under a pressure of 200 atmospheres or more. In the above-mentioned method, the corundum particles can be produced by a treatment under a very high pressure of 200 atmospheres or more, and this high pressure causes the resultant corundum plate crystalline particles to have a large thickness of 1 .mu.m or more, and it is difficult to orient these very thick particles in the intended article.
In another method, the plate crystalline corundum particles can be produced by dry heat-treating aluminum oxide particles or aluminum hydroxide particles, but the particles obtained by the above-mentioned method have a large particle size of 10 .mu.m or more, and thus it is difficult to firmly bond them together.